In general, in massive structures such as electrical power equipment, gas tank, a bridge for vehicles, a high-speed train station building on top of which high-speed trains are driven, general building structures, etc., an upper structure and a lower structure are formed separately in order to guarantee reliability of the structures against earthquakes or vibrations, and a seismic isolator is installed between the upper and lower structures for supporting elastically the weight of the upper structure and reducing vibrations in vertical and/or horizontal directions.
Conventional seismic isolators were disclosed in a Korean Patent Publication KR 10-1210656 (title: Seismic isolator for electrical equipment for controlling vibrations along 3 axes, inventor: Sanghoon Oh). This Oh patent had problems: since weight of upper structure was supported by the force of spring only the vertical vibration reduction is small, since a compressed state must be maintained for a long time it may cause a problem in using for an extended period of time, and after a horizontal displacement by earthquakes it cannot return horizontally to an original position.
And, other types of seismic isolator using wedge was disclosed in another Korean Patent Publication KR 10-0716088 (title: Vertical Vibration Reducing Bearing using friction, inventor: Youngcheol Cho). The seismic isolator disclosed in the Cho patent is adapted for supporting a highly massive structure with very large weight such as an upper structure of a bridge, but is not appropriate to support electrical power equipments, which have less weight than the highly massive structure and include relatively weak materials, for some shock-absorbing in up and down directions and/or horizontal directions.